The present invention relates to ultrasound systems which image anatomical structures and the movement thereof. More particularly, the present invention relates to a method and apparatus for accumulating and storing a complete set of ultrasound information from a region of interest during a scanning period and then, in a post-scanning operation, processing the stored ultrasound information to provide a number of various and selectable analysis and display modes.
Doppler ultrasound systems rely on the Doppler effect to detect movement by measuring the change in frequency between a transmitted ultrasound signal and the retuning echoes. If it is necessary or desirable to limit Doppler analysis only to echoes returned from a structure at a known depth, pulsed ultrasound is employed. Pulsed ultrasound allows the time for the ultrasound signal to make a round trip from the transmitter to the target and back to the receiver to be measured and the depth of reflecting structures calculated. In pulsed Doppler systems the operator has the opportunity of determining the depth from which Doppler signals are to be collected. In practice this is done by selectively ignoring signals returning to the receiver until a selected time interval after transmission of the ultrasonic pulse. The receiver is then switched on for a further short interval, during which Doppler information is collected. The duration of this collection interval determines the length of the data collection volume within the tissue. The sensitive zone created by this technique is commonly referred to as the "range gate" or "Doppler gate".
Spectral Doppler uses pulsed Doppler techniques to measure the velocity of targets, such as blood cells within a vessel, at a predefined depth. Usually a two-dimensional B-mode ultrasound image is used to locate the vessel of interest. The system operator then sets the Doppler gate to correspond to the location (depth) and width of the vessel along the appropriate ultrasound beam or scan line. Once the Doppler gate is set, a number of clinically useful analyses can be made. For example, spectrum analysis of the Doppler shift frequencies provides information regarding the range of different velocities within a vessel. A blockage or stenosis within a blood vessel, for example, will create a wider range of velocities and, therefore, a broader spectrum of Doppler shift frequencies would be observed than in the case of a healthy vessel. A quantitative velocity analysis can be made if the angle between the ultrasound beam and long axis of the vessel is known. Many conventional ultrasound systems permit the operator to set the beam/vessel angle by tracing a line along the axis of the vessel under examination.
Because of the rapidity and transient nature of abnormal blood flow patterns and other movements such as cardiac contractions, Doppler ultrasound systems may use recording systems to store a series of images. These images may then be played back at slow speed or frame by frame in a post-scanning operation. Video recorders or a digital memory (often referred to as a "cine loop") capable of recording a few seconds worth of images are incorporated into many conventional ultrasound systems. The information stored by and played back from a typical cine loop is generally limited by the analysis being performed during recording. The reason for this limitation is that a conventional cine loop receives data produced after the echo signals have been processed and prepared for display. Therefore, the cine loop stores only the data resulting from a particular processing operation carried out upon the echo signals. The processing operation is determined by the present mode of operation and parameter settings. The processed data may ignore and/or eliminate certain information from the echo signals. For example, if color flow imaging were being performed on one sub-region within a region of interest, the only information that is stored and available for playback may be the same color flow image from the same sub-region. Similarly, post-scanning playback of a spectral Doppler analysis is limited by the Doppler gate location and width set prior to initiating the cine loop recording. Information contained in echoes received outside the Doppler gate "window" or along non-selected scan lines is ignored and, therefore, lost forever. Also, the accuracy and usefulness of a quantitative velocity measurement would depend on the beam/vessel angle traced during the original scan.
The above mentioned limitations of known cine loop schemes lead to several disadvantages. For example, each time a different kind of Doppler analyses is undertaken, a different Doppler gate location or width is set or a different sub-region is selected for color flow imaging, an additional scanning period must be initiated and new information must be stored in the cine loop. Analyses of different structures at multiple gate locations at the same moment in time is not possible. Also, an abnormality recognized in a recorded image after the patient has left, cannot be analyzed in greater detail unless the patient returns for a new scanning session (and then the abnormality present during the original scanning session may not reveal itself). Images that are recorded while inaccurate or less than optimal parameters are set may be useless. Anything that increases the length or number of ultrasound scanning sessions, may increase patient exposure time, patient discomfort and procedure costs. Furthermore, studies employing contrast agents are limited in the number of different analyses that can be performed during the rapid decay of the contrast agent.
A need remains for an improved ultrasound system to overcome the above-identified difficulties and limitations. It is an object of the present invention to meet this need.